*** September 2019 : It appears that the configurations mentioned here no longer work unfortunately.
One of the great things about SharePoint Online is that you can create custom columns for just about every element in a site. The information that SharePoint Online displays to you can be customised using “Views”. You can configure multiple “Views” inside a List or Library to show exactly the information you want, as you can see above.
You can also adjust the width of any column by simply dragging it out like you do in something such as Excel. Just go to the boundary of the column heading and drag the column width out as shown above.
The issue was that when you return to this layout after the current session (i.e. closed the browser and logged in again later), the column width would revert back to the default width. Frustrating.
Also, if another user looked at the same location they would again, only see the default widths, even though you changed it in your session.
So, any column changes made were not persistent between sessions or globally available. Frustrating.
However, if you take a close look at the “View” pull down in the top right of the page you will notice something.
You should see that once you change a column width on the page a * appears after the “View” name.
Again, make a change to the column width and a little * will appear. This is telling you that the “View” has changed is different from the default. This is kinda the same when you edit documents. It indicates that the contents have changed and you SHOULD save these changes for them to be retained!
If you therefore select the “View” pull down there will be an option to Save view as.
If you then Save the “View”, using the same name as before, you are overviewing the old display with the new layout i.e. with different column widths.
If another user now navigates to the page they will see the columns widths that you set! i.e. saving the “View” after adjusting the columns sets these columns width for all users! Yes Martha, we have achieved the global configuration for column widths in SharePoint Online! Yeah!
In summary then, adjust the column widths to the size you want and then save the “View” to make those widths available to all users globally.